Y-Bi-Ba-Cu-O and Bi-Pb-Sr-Ca-Cu-O superconductors in disk-form have been prepared by conventional ceramic powder compacting method. X-ray powder diffraction patterns were taken to analyse crystal structure and phases. Photographs of surfaces were taken using a scanning electron microscope to study microstructure. Energy dispersive analysis of X-ray(EDAX) data and Xray mapping data were also taken to investigate the macroscopic and microscopic variations of composition. The electrical resistance was measured by 4-point probe method using silver contacts and diamagnetic property was observed by levitation method.
From the analyses of D.T.A. data and X-ray diffraction patterns, calcination of 7 hrs at 850 °C in nitrogen was determined to be the optimum conditions for the calcination of materials in the Y-Bi-Ba-Cu-O system.
Sintering temperature, in the ranges from 850°C to 950°C, affects the value of electrical resistivity in normal state, its effects on magnetic properties is negligible. The electrical resistivity in normal state is smaller in $Y_{0.85}Bi_{0.15}Ba_2Cu_3O_{7-y}$, where the Bi is substituted into Y site than in basic compound $Y_1Ba_2Cu_3O_{7-y}$ due to improved microstructure, i.e. the substitution of Bi into the $Y_1Ba_2Cu_3O_{7-y}$ enhances the sinterability and reduced the electrical resistivity by increasing the contact area between grains. However, the effects of presence of 0.5 and 2 w/o $Bi_2O_3$ in calcined $Y_1Ba_2Cu_3O_{7-y}$ powder is to decrease Tc, when sintered above 950°C, and to increase the normal state electrical resistivity by the formation of microvoids between the grains.
The fact that Y site is substituted with Bi is identified indirectly by X-ray diffraction patterns and by semi-empirical method which compares the theoretical minimum anion-cation interatomic distance calculated by ion size and electrostatic interactions with practical neutron diffraction data of $Y_1Ba_2Cu_3O_{7-y}$ system.
Sample with composition of ($Bi_{0.7}Pb_{0.3}Sr_1Ca_1Cu_{1.8}O_x$ were sintered by various conditions. Firstly, samples were sintered for 7days in air at various temperatures. The electrical resistance decreases sharply in two steps as the temperature decreases for the specimen that was sintered at 840°C and almost in one stop for the specimens that was sintered at 850°C. As the sintering temperature is increased to 855°C, the sample does not show superconducting properties down to 77K.
The microstructures of the sintered specimens consisted of two types of grains:rod-like grains were imbedded in matrix consisted of platelet grains for the specimens sintered up to 850°C and platelet grains were imbedded in matrix consisted of rod-like grains for the specimen that was sintered at 855°C. This fact combined with the resistance vs. temperature curve indicates that the superconducting material is consisted of platelet grains. The amount of non-superconducting rod-like grains((Sr, Ca)-Cu-$O_x$) appears to be related with the melting of $Bi_2O_3$ (Tm: 854°C).
Secondly, a number of samples were sintered at 845°C for various periods. As the sintering period increases, the amount of high Tc phase increases accompanying the decrease in low Tc phase while the amount of rodlike grains is independent of the sintering period, resulting in a superconductor with Tc of 100K. Change in composition of low, high Tc phase and (Sr, Ca)-Cu-$O_x$ also occur during sintering due to evaporation of Bismuth and lead. This facts are identified by EDAX and X-ray mapping to investigate the macroscopic and microscopic variations of initial composition $(Bi_{0.7}Pb_{0.3})Sr_1Ca_1Cu_{1.3}O_x$. One of the major role the PbO plays during the fabrication process is to form $Ca_2PbO_4$ phase at early stages of sintering which subsequently acts as a catalyst to form the high Tc phase out of low Tc phase.
The rate of the formation of the high Tc phase can further be increased by resintering method: sintering below Tm of $Bi_2O_3$ to obtain proper microstructure with $Ca_2PbO_4$ phase and then resintering above the Tm to enhance the formation rate of the high Tc phase. Thus it is possible to fabricate $(Bi_{0.7}Pb_{0.3})Sr_1Ca_1Cu_{1.8}O_x$ superconductors with Tc of 106K by sintering a 845°C for 48 hrs followed by sintering of 24 hrs at 855°C.
